1. Field of the Invention
The present invention relates to an engine control apparatus for controlling a target engine of the type having an output shaft rotatably driven to generate a torque on the output shaft through combustion of fuel inside a cylinder of the engine.
2. Description of the Related Art
As is well known, in an engine (internal combustion engine) used as a power source for a motor vehicle, for example, fuel supplied by injection from an appropriate fuel injection valve (such as an injector) is ignited and combusted to generate an output torque. In recent years, automotive diesel engines have employed a so-called “multistage injection system” in which a main injection for generating the output torque in a single combustion cycle is either preceded or followed by a sub-injection performed with a smaller injection quantity than the main injection. This is due, for example, to a problem of increased emission of noise and NOx during combustion, which has attracted special interest recently. In such an automotive diesel engine equipped with the multistage injection system, a pre-injection or a pilot injection with small injection quantity is performed before the main injection so as to improve the aforesaid problem. Furthermore, the main injection is followed by either an after injection (performed at injection timing coming close to the main injection) for the purpose of activating diffusion combustion to thereby reduce PM (particulate matter) emissions, or a post injection (performed at injection timing with a large lag angle provided relative to the main injection) for the purpose of increasing the engine exhaust temperature and supplying a reductive component to thereby activate a catalyst. In a sophisticated engine control system, the supply of fuel to the engine is performed by combining the aforesaid various forms of injection in an appropriate manner so as to achieve an injection pattern well suited for each of the various supposed conditions.
In most cases, however, the injection quantity of the sub-injection is very small and, hence, in performing such sub-injection, all accurate control of the injection quantity becomes particularly important. To address this problems, improved engine control apparatuses have been proposed, which are arranged to variably control a pilot injection quantity based on a heat generation quantity per unit time (instantaneous heat generation quantity), such as disclosed in Japanese Patent Laid-open Publications (JP-A) Nos. 11-148410 and 11-141386. In the disclosed apparatus, a change in instantaneous heat generation quantity is determined and, based on the determined change and more specifically a maximum value (peak) of the determined change, the pilot injection quantity is variably controlled. In other words, the pilot injection quantity control in the disclosed apparatuses is performed so that each individual instantaneous heat generation quantity (or a peak thereof) matches a desired heat generation quantity to thereby reduce noise and NOx emissions during combustion.
In the disclosed prior apparatuses, however, a pilot injection quantity is injected in accordance with a desired heat generation quantity (target heat generation quantity). Accordingly if the pilot injection is performed in a hostile environment or a certain injection condition in which the desired heat generation quantity is difficult to obtain, it is likely to occur that an excess pilot injection quantity is injected with an aim to meet the desired heat generation quantity. In this instance, however, because the penetration length increases directly with the injection quantity, fuel spray or mist can reach a cylinder wall surface (or a pipe inner wall surface) before full vaporization and is allowed to adhere to the cylinder wall surface. Disadvantageously, such fuel adhesion will increase the amount of unburned fuel (HC) and cause deterioration of emissions and fuel consumption rate.